Chirality, or "handedness", refers to a property of structures that exist in two versions – “enantiomers” - that are mirror images of each other but cannot be superimposed. The chirality of molecular structures can significantly affect how a substance affects biological systems and an example of such a molecule is penicillamine, where the right-handed version can be used to treat rheumatoid arthritis, whereas the left-handed version is toxic. Another example is thalidomide, which in its R-form acts as a pain-killing drug, while its S-form causes major birth defects.

Plasmonic metamaterials are arrays of tiny metal structures that can be fine-tuned to interact with light in very specific ways. With these materials, it is possible to tune the shape, size and arrangement of the structures to support collective oscillations of conduction electrons – called plasmons – at specific frequencies.

The chiral versions of plasmonic metamaterials are promising for applications in chiral sensors and photonics devices thanks to their strong optical chirality and the fact that they strongly interact with light at the subwavelength scale. However, most plasmonic chiral metamaterials today are either chiral plasmonic nanostructures or anisotropic achiral plasmonic nanostructures stacked into complex chiral architectures with site-specific twists. This means that they often need to be made using sophisticated lithographic techniques, such as e-beam lithography and focused ion-beam lithography, which limits their practical applications. What is more, it is difficult to make tuneable structures using such techniques.

Simplifying chiral metamaterials

The new chiral plasmonic metamaterials made by Zilong Wu and Yuebing Zheng are much simpler in that they are made by stacking just two layers of identical achiral gold nanohole arrays on top of each other.

The researchers made their plasmonic chiral metamaterials (MCMs) by first using conventional nanosphere lithography to define periodic nanohole arrays in each gold layer (that had been deposited by electron-beam evaporation). The top gold layer was prepared on a copper substrate so that it could then be transferred to the bottom layer using a routine wet chemistry transfer technique.

“Our chiral metamaterials show strong optical chirality, despite being only around 70 nm thin,” explains Wu. “The chiroptical response of these structures can be precisely tuned by simply rotating the two layers in-plane with respect to each other to produce so-called moiré patterns.” These twists cause dramatic changes in the structure’s electronic properties.

“Plasmonic polarimetry”

“These structures can also discriminate between right- and left-handed proteins as well as between R- and S-thalidomide molecules at the picogram level in solution,” he tells nanotechweb.org. “We are able to perform these sensing measurements by exploiting the strong super-chiral fields generated by the MCMs. This mechanism is based on ‘plasmonic polarimetry’ where chiral molecules adsorbed on the surface of the metamaterial can introduce asymmetric modifications in its local refractive index. These modifications then induce asymmetric peak or dip shifts in the circular dichroism spectra of the MCMs depending on whether the molecule being sensed is left- or right-handed respectively.

“Such sensing might of crucial importance in the medical industry because the ‘wrong’ enantiomers of many chiral drug molecules produce harmful effects in the human body, so it is vital to be able to distinguish between these and the harmless ones,” he adds.

Spurred on by their preliminary results, Wu and Zheng say that they are now looking to improve the large-scale uniformity in their metamaterials during fabrication. They will be trying out techniques other than nanosphere lithography, which they admit might not be the best process here since it is not as precise, as say, nanoimprint lithography or injection moulding. “These techniques might allow us to make MCMs with single domains and precisely controlled rotation angles on the large scale,” says Wu.

The research is detailed in Advanced Optical Materials DOI: 10.1002/adom.201700034.